The present invention relates to a method for measuring the distance of targets in the surroundings by way of a time-of-flight measurement of pulses reflected at said targets. The pulses can be of any type, for example light pulses, in particular laser pulses, radio pulses, in particular radar pulses, sound pulses, or the like.
Modern pulse time-of-flight distance meters, such as laser distance meters or scanners, work with high pulse power over large distances and/or high pulse repetition rates in order to quickly create a number of distance measurement points in the surroundings, wherein they attain a high time and/or spatial resolution. In both cases the situation can arise that the next pulse is already emitted before the reflection of the last pulse has been received, so that the incoming received pulses can no longer be clearly assigned to their corresponding transmitted pulse. This is known as a “multiple time around” (MTA) or “multiple pulses in the air” problem. The maximum size dmax of the distance range that can be clearly measured, or what is known as the MTA zone, is given here from the pulse repetition rate (PRR) and the light speed c to give dmax=c/(2·PRR).
If a laser scanner for example offers a pulse repetition rate of 400 kHz, this corresponds to an MTA zone size dmax von of approximately 375 m.
A wide range of methods is known for correct mutual assignment of the transmitted and received pulses for clear distance measurement results. A first possibility lies in ensuring during the planning of the measurement task that all anticipated targets in the surroundings lie within the same MTA zone so as to be able to perform the correct assignment. This method is naturally applicable only for specific measurement tasks, and for example is unsuitable for highly mobile or long-range measurement or scanning tasks, for example the air-borne scanning of topographies or the ground-borne measurement of mountain ranges or moving vehicles.
Another group of methods is based on the principle of making the individual transmitted pulses distinguishable from one another by varying and/or coding their polarisation, amplitude or wavelength, so as to be able to assign the received pulses accordingly. These methods, however, are either suitable only for few “pulses in the air” or require complex coded pulses, which limits the pulse repetition rate and the measurable distance range and extends the measurement time.
An alternative method which uses pulse position modulation is known from patent EP 2 694 996 B1 in the name of the same applicant. Here, a sequence of pulses modulated in respect of their mutual pulse intervals is emitted, and the correct MTA zone is determined by searching for the least noisy received pulse sequence.
All of these methods, however, have the common feature that the receipt of a reflected (earlier) pulse at the time of emission of a (later) pulse is not possible due to design, more specifically because the receiving electronics is saturated or overloaded by near reflections or the back-scattering of an emitted pulse at components of the distance meter, for example housing or assembly parts or an exit window thereof, and is thus “blind” to the receipt of a reflected pulse. A reception gap or what is known as a “blind range” thus remains in the distance measurement at each boundary between two MTA zones. Reflections at targets in the surroundings at such a distance cannot be received at all—or in the case of pulse position modulation can only be received in individual cases and therefore only with very low time and/or spatial resolution.